A Sealed Two-Level Metal System using Tungsten

A sealed two-level metallization system using tungsten has been developed at Bell Laboratories for LSI MOS fabrication. Tungsten is used-for both gate and interconnecting metal and is sealed from corrosive environments by an upper layer of Si3N4. Beam leads are provided at the periphery through via holes in the nitride for circuit access. Accelerated aging tests were performed to evaluate the chemical, mechanical and electrical stability performance of this system using both test chips and an actual 1024 bit dynamic RAM. Results from tests on packaged ceramic DIPs coated with RTV rubber indicate >300 hrs. MTTF for steam bomb tests at 55 psi and 150°C, and >108 hrs. MTTF at 80°C for high temperature reverse bias tests. Dynamic step stress results give a lower bound of 106 hrs. at 80°C. These results are within the requirements of most high reliability systems and indicate that tungsten is a viable alternative to aluminum.